A variety of techniques are currently utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including, among other things, the resistivity, porosity, and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD). One advantage of MWD techniques is that the information about the rock formation is available at an earlier time when the formation is not yet damaged by an invasion of the drilling mud. Thus, MWD logging may often deliver better formation evaluation (FE) data quality. In addition, having the formation evaluation (FE) data available already during drilling may enable the use of the FE data to influence decisions related to the ongoing drilling (such as geo-steering, for example). Yet another advantage is the time saving and, hence, cost saving if a separate wireline logging run can be avoided.
For an accurate analysis of some FE measurements, for example, neutron porosity (NP) measurements and/or neutron density (ND) measurements, and the like, it is important to know the actual downhole formation evaluation (FE) tool position in a borehole during drilling. By way of example, an 8-sector azimuthal caliper with 16 radii allows the determination of the exact center of the downhole formation evaluation (FE) tool in the borehole during drilling and a magnetometer allows the determination of the exact orientation of the detector face. These two parameters allow optimization of the environmental borehole effects, such as correction for borehole size and mud.
However, conventional corrections typically assume one of two conditions. Either (1) the downhole formation evaluation (FE) tool is eccentered (the FE tool center is eccentrically located with respect to the “true” center of the borehole and the FE tool center does not coincide with the true center of the borehole), and appropriate eccentered FE tool corrections are used, or (2) the downhole formation evaluation (FE) tool is centered (the FE tool center is not eccentrically located with respect to the true center of the borehole and the FE tool center does coincide with the true center of the borehole) and appropriate centered FE tool corrections are used.
In the eccentered case, conventionally an average eccentered correction for constant rotation of the FE tool is assumed whereby the FE tool is assumed to face the formation about 50% of the time and to face into the borehole about 50% of the time. However, the conventional approaches are not able to allow the selection of the proper environmental corrections to apply generally, lacking any way to track the FE tool center and direction with respect to the borehole center. For a non-azimuthal FE tool, for example, the conventional approaches lack any way to extrapolate between (1) the eccentered and (2) the centered cases described above, even assuming constant FE tool rotation. Similarly, for an azimuthal FE tool, where the diluting effects of the FE tool rotation may not appear, the conventional approaches lack any way to provide an absolute standoff correction correcting for the FE tool location and orientation in the borehole.
While it has long been known that two-way travel time of an acoustic signal through a borehole contains geometric information about the borehole, methods of efficiently obtaining that geometric information acoustically continue to need improvement. In particular, a need exists for efficient ways to obtain such geometric information about a borehole to overcome, or at least substantially ameliorate, one or more of the problems described above.